The present invention relates to an electrode for a polymer electrolyte fuel cell and a method of producing the same.
In general, the electrode for the polymer electrolyte fuel cell is produced by coating a porous conductive electrode substrate with a carbon fine powder carrying a noble metal.
In the production of such electrodes, when the electrode is directly formed on the surface of polymer electrolyte by means of printing, a large area where the electrode reaction takes place can be secured. It is however very difficult to form an electrode on the polymer electrolyte film in viewpoint of a swelling property or fastening property of the polymer electrolyte film.
For that reason, these electrodes are usually produced by coating the electrode substrate employing the screen printing process or transfer printing (offset) process with an ink prepared by dispersing the carbon fine powder carrying the noble metal in an organic medium such as isopropyl alcohol.
In a case of using an organic medium as the dispersing medium for the ink, a protecting gear for operators, an exhausting equipment and the like have to be used for a safety measure. In recent years, the organic medium has therefore been replaced by an aqueous medium which has a higher safety.
To the ink using either of the organic medium or the aqueous medium, an ionic or nonionic surfactant has previously been added in the ink for improving a dispersibility of the carbon fine powder carrying noble metal.
If the organic medium or the surfactant used during the production of the electrode remains in the produced electrode, it deteriorates the performance of the assembled fuel cell. In a case of producing the electrode by using the organic medium, it is necessary to remove the organic medium in the coated film on the electrode substrate by drying the film in viewpoint of securing safety in the subsequent production steps or that of deterioration in cell performance due to the remained medium.
In addition, to the ink, a pore-producing agent has previously been added for forming micropores in the electrode so that diffusion of gas should not be prevented in the electrode. The pore-producing agent included in the formed layer of electrode must be removed by baking or washing.
Alternatively, there is a method for producing a film electrode by coating a resin film with a slurry of the catalyst powder by means of the doctor blading process or the like. The film electrode thus obtained is adhered on the polymer electrolyte film by hot pressing or the like. In this method, there is also a need for using a pore-producing agent or a surfactant.
As described above, the production of the electrodes for the polymer electrolyte fuel cells has hereto been very man-power consuming operation. In addition, a closest attention has been required for the safety measure.
The present invention intends to solve the above-mentioned problems in the conventional production of the electrode for the fuel cell. An object of the present invention is to provide a method of producing an electrode having a high performance in a simple and convenient manner, which does not require the use of any of the medium, surfactant and pore-producing agent.
The method of producing an electrode for a fuel cell in accordance with the present invention comprises the step of applying an electrostatically-charged catalyst powder onto a surface of a polymer electrolyte film or a porous conductive electrode substrate to form a layer including the catalyst powder on the polymer electrolyte film or the porous conductive electrode substrate.
For instance, the electrostatically-charged catalyst powder is supplied to the vicinity of the polymer electrolyte film or porous conductive electrode substrate which is grounded, thereby causing the catalyst powder to be absorbed on the surface of the polymer electrolyte film or the porous conductive electrode substrate by an electrostatic force Alternatively, the electrostatically-charged catalyst powder is adhered to a roll, and the adhered catalyst powder is then offset-printed onto the polymer electrolyte film or the porous conductive electrode substrate.
Another method of producing an electrode for a fuel cell in accordance with the present invention comprises the step of spraying a catalyst powder together with a carrier gas onto a surface of a polymer electrolyte film or a porous conductive electrode substrate to adhere the catalyst powder to the surface, thereby to form a layer including the catalyst powder on the surface of the polymer electrolyte film or the porous conductive electrode substrate.
For spraying the catalyst powder, for instance, a spray gun is employed. By appropriately controlling the spray gun, it is possible to uniformly form an electrode having a large area.
In a case of spraying the catalyst powder onto the polymer electrolyte film, in particular, particles of the catalyst powder are embedded in the film. By this procedure, the cell performance is further improved.
In this method also, it is preferable to use the electrostatically-charged catalyst powder.
Since the method of producing the electrode in accordance with the present invention is a fully dry process, there is no need for employing an organic medium. Therefore, safety in the operation is improved. In addition, a drying step for removing the organic medium remaining in the formed electrode can be dispensed with. Further, according to the method of the present invention, there is no need for employing any of surfactants or pore-producing agents. Therefore, a step of removing these can also be dispensed with. In addition, the method is superior to any conventional methods in economical viewpoint, because it is possible to recover the catalyst powder which was not utilized for forming the electrode with ease.
In addition, the formation of the electrode on the polymer electrolyte film, which had heretofore been difficult, can easily be performed without any concern for the swelling or the like disadvantages. Although the smoothness and density of the electrode film obtained in accordance with the method of the present invention are inferior to that obtained by printing, the gas-diffusion property of the electrode is conversely improved, and thus the present method is superior to the printing process as the production method for an electrode.
In addition, the performance of the fuel cell configured with the electrode of the present invention is superior to those of the fuel cells configured with the conventional electrodes.
The catalyst powder includes a carbon fine powder carrying a noble metal such as platinum, ruthenium, gold, palladium or the like. In addition, it is preferable that the catalyst powder further includes a carbon fine powder coated with a polymer electrolyte or a fluorocarbon resin such as polytetrafluoroethylene. The electrode layer may be configured by laminating layers including various catalyst powders.
The catalyst powder is preferably prepared by mixing a carbon fine powder carrying a noble metal with a colloidal dispersion of the polymer electrolyte and then drying the mixture.
In a preferred mode of the present invention, the method includes the further step of heating the layer of catalyst powder and the polymer electrolyte film to further the adhesion of the catalyst powder to the polymer electrolyte film. By this further heating step, an electrode having higher performance is obtained. More preferably, the method further comprises, in advance of the heating step, the steps of applying a polymer electrolyte solution on the surface of the polymer electrolyte film and then drying to evaporate the solvent from the solution.
According to the method of the present invention, it is possible to produce an electrode for a fuel cell without using any solvent or surfactant which is considered to impose an adverse influence on the cell performance. In addition, since an electrode structure being preferable for the gas diffusion can be realized without employing the pore-producing agent, the product is an optimum as the electrode for the fuel cell.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.